Method and apparatus for compensating for position slip in interface devices

ABSTRACT

Method and apparatus for compensating for position slip in interface devices that may occur between a manipulandum and a sensor of the device due to a mechanical transmission. A device position delta is determined from a sensed position of a manipulandum of an interface device. It is determined if position slip has occurred caused by a change in position of the manipulandum that was not sensed by a sensor of the interface device, typically caused by a mechanical transmission between sensor and manipulandum. If position slip has occurred, an error in the sensed position caused by the position slip is corrected by adjusting the sensed position to take into account the position slip. The adjusted position delta is used as the position of the manipulandum and the display of objects controlled by the interface device are accordingly compensated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/133,586, filed May 11, 1999 by Braun et al., which is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to sensing techniques forhuman-computer interface devices.

Using an interface device, a user can interact with an environmentdisplayed by a computer system to perform functions and tasks on thecomputer, such as playing a game, experiencing a simulation or virtualreality environment, using a computer aided design system, operating agraphical user interface (GUI), or otherwise influencing events orimages depicted on the screen. Common human-computer interface devicesused for such interaction include a joystick, mouse, trackball, steeringwheel, stylus, tablet, pressure-sensitive ball, or the like, that isconnected to the computer system controlling the displayed environment.

In some interface devices, haptic feedback is also provided to the user,also known as “force feedback.” These types of interface devices canprovide physical sensations which are felt by the user manipulating thephysical object of the interface device. For example, the Force-FXjoystick controller from CH Products, Inc. or the Wingman Force joystickfrom Logitech may be connected to a computer and provides forces to auser of the controller. Other systems might use a force feedback mousecontroller. One or more motors or other actuators are used in the deviceand are connected to the controlling computer system. The computersystem controls forces on the force feedback device in conjunction andcoordinated with displayed events and interactions on the host bysending control signals or commands to the force feedback device and theactuators.

In both force feedback devices and non-force feedback devices, it isimportant to accurately sense the position of the manipulandum moved bythe user so that interactions displayed on the screen are accuratelyportrayed. Furthermore, the determination of forces in force feedbackdevices often depends on the position of the manipulandum so thataccurate sensed position is critical for realistic force feedback.

Thus, all interface devices require some sort of position sensor (orsensors) in order to read the input position of the manipulandum (“endeffector”). Sometimes the position sensors are connected directly to theend effector itself. However, in other cases the sensor is actuallyconnected through a mechanical transmission to the end effector. Forexample, a gear transmission, capstan drive (cable) transmission, orbelt drive transmission can be used. The purpose of the transmission, asfar as the position sensor is concerned, is to amplify the range ofmotion that the sensor can detect and allow the use of lower resolutionsensors, which are typically less expensive. A mechanical transmissionmay also be used to amplify forces output by the actuators of the forcefeedback device. The transmission for a sensor can be the sametransmission as an actuator uses; several different joystick devices usethis method, such as the device described in copending patentapplication Ser. No. 09/138,304, which provides a sensor connected to anactuator shaft. In other devices, an independent mechanical transmissiondedicated to the sensor can be used.

In either of these cases, there is the possibility of position slipoccurring in the transmission, where the end effector moves somedistance without the sensor detecting that movement or inaccuratelydetecting that movement; e.g., a gear skips teeth, a belt or cable slipson a pulley, etc. When a relative position sensor is used in the device,such as a quadrature encoder, this position slip should be detected orthe device will be unable to determine an accurate position for the endeffector when slip occurs. The problem is amplified for force feedbackdevices because they often operate in a limited workspace and depend onthe ability to determine the absolute device position in the limitedworkspace. Furthermore, some forces output by the device are based on anaccurate position of the manipulandum, so that the forces can becomediscontinuous or distorted if position slip occurs.

SUMMARY OF INVENTION

The present invention provides a method and apparatus for compensatingfor position slip in interface devices that may occur between amanipulandum and a sensor of the device due to a mechanicaltransmission. More accurate sensing of manipulandum position and smoothdisplay of controlled objects based on manipulandum position areachieved.

More specifically, a method for correcting for slippage in a positionsensing system of an interface device coupled to a host computerincludes determining a device position delta from a sensed position of amanipulandum of the interface device in a degree of freedom. Theposition delta indicates a change in position of the manipulandum. It isdetermined if position slip has occurred in the position sensing system,where the position slip is caused by a change in position of themanipulandum that was not sensed by a sensor of the interface device.The position slip is typically caused by a mechanical transmissionprovided between the sensor and manipulandum, such as a gear drive, abelt drive, and a capstan drive. If position slip has occurred, an errorin the sensed position caused by the position slip is corrected byadjusting the sensed position to take into account the position slip.The adjusted position delta is used as the position of the manipulandum.

Preferably, the determining if the slip has occurred includes checking areference signal from a reference sensor included on the interfacedevice, the reference sensor detecting a position of a referencelocation coupled to the manipulandum. A computation value is preferablydetermined from the device position delta, where a display of the hostcomputer is updated based on the computation value. In one method, thecomputation value is not adjusted with the device position delta whenthe position slip is detected, allowing smoother display of graphicalobjects. In some embodiments, a force having a magnitude and directionis determined at least partially based on the computation value, andwhere the force is output by an actuator included in the interfacedevice. Preferably, the computation value is modified to compensate foran offset between device position and screen position that may occur dueto the position slip and the adjustment of computation value. Forexample, a user-controlled object displayed by the host computer can beallowed to continue to be moved after the manipulandum has reached alimit to a workspace of the manipulandum, where the user-controlledobject continues to be moved in a direction corresponding to a directionof the manipulandum into the limit.

In another method, the determining of a device position, determining ifa position slip has occurred, and correcting an error are performed overseveral iterations, and the sensed position is adjusted or corrected byan amount which gradually reduces the error in the sensed position overthose iterations. The adjusting of the sensed position in this fashionis preferably not performed if the manipulandum is not moving in thedegree of freedom.

In another aspect of the present invention, an interface device providesinput to a host computer based on manipulations of a user. The interfacedevice includes a manipulandum physically contacted by the user andmovable in at least one degree of freedom. A position sensor detects aposition of the manipulandum in the degree of freedom. A mechanicaltransmission is coupled between the manipulandum and the positionsensor, and a reference sensor detects the motion of at least onereference location coupled to the manipulandum, the reference sensorproviding a reference signal used to determine if position slip hasoccurred between the manipulandum and position sensor. One or moreactuators can be provided to output forces for force feedback devices. Alocal microprocessor can receive the reference signal and adjust thedetected position of the manipulandum to compensate for the positionslip. The local microprocessor can also determines a computation valuefrom a position delta, where the computation value is adjusted if theposition slip has occurred. The computation value provided to the hostcomputer to update a display of the host computer and can be used toupdate output forces and other values.

The present invention advantageously provides a sensing system thattakes advantage of the increased sensing resolution offered by the useof mechanical transmissions such as gear drives and belt drives, and yetaccurately compensates for the inaccurate sensing that may occur whenusing such transmissions due to position slip.

These and other advantages of the present invention will become apparentto those skilled in the art upon a reading of the followingspecification of the invention and a study of the several figures of thedrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of system including an interface device andhost computer for use with the present invention;

FIG. 2 is a top plan view of an example of a force feedback interfacedevice suitable for use with the present invention;

FIG. 3 is a perspective view of a reference sensor of the presentinvention for detecting position slip;

FIG. 4 is a flow diagram illustrating a first method of the presentinvention for compensating for position slip in interface devices;

FIG. 5 is a flow diagram illustrating a method for detecting positionslip and the amount of slip; and

FIG. 6 is a flow diagram illustrating a second method of the presentinvention for compensating for position slip in interface devices.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a block diagram illustrating a force feedback interface system10 suitable for use with the present invention controlled by a hostcomputer system. Interface system 10 includes a host computer system 12and an interface device 14.

Host computer system 12 can be a personal computer, such as anIBM-compatible or Macintosh personal computer, or a workstation, such asa SUN or Silicon Graphics workstation. Alternatively, host computersystem 12 can be one of a variety of home video game systems, such assystems available from Nintendo, Sega, or Sony, a television “set topbox” or a “network computer”, etc. Host computer system 12 preferablyimplements a host application program with which a user 22 isinteracting via peripherals and interface device 14. For example, thehost application program can be a video or computer game, medicalsimulation, scientific analysis program, operating system, graphicaluser interface, or other application program that utilizes forcefeedback. Typically, the host application provides images to bedisplayed on a display output device, as described below, and/or otherfeedback, such as auditory signals.

Host computer system 12 preferably includes a host microprocessor 16, aclock 18, a display screen 20, and an audio output device 21.Microprocessor 16 can be one or more of any of well-knownmicroprocessors. Random access memory (RAM), read-only memory (ROM), andinput/output (I/O) electronics are preferably also included in the hostcomputer. Display screen 20 can be used to display images generated byhost computer system 12 or other computer systems, and can be a standarddisplay screen, CRT, flat-panel display, 3-D goggles, or any othervisual interface. Audio output device 21, such as speakers, ispreferably coupled to host microprocessor 16 via amplifiers, filters,and other circuitry well known to those skilled in the art (e.g. in asound card) and provides sound output to user 22 from the host computer12. Other types of peripherals can also be coupled to host processor 16,such as storage devices (hard disk drive, CD ROM/DVD-ROM drive, floppydisk drive, etc.), printers, and other input and output devices. Datafor implementing the interfaces of the present invention can be storedon computer readable media such as memory (RAM or ROM), a hard disk, aCD-ROM or DVD-ROM, etc.

An interface device 14 is coupled to host computer system 12 by abi-directional bus 24. The bi-directional bus sends signals in eitherdirection between host computer system 12 and the interface device. Aninterface port of host computer system 12, such as an RS232 or UniversalSerial Bus (USB) serial interface port, parallel port, game port, etc.,connects bus 24 to host computer system 12. Alternatively, a wirelesscommunication link can be used.

Interface device 14 can include a local microprocessor 26, sensors 28,actuators 30, a user object 34, optional sensor interface 36, anoptional actuator interface 38, and other optional input devices 39.Local microprocessor 26 is coupled to bus 24 and is considered local tointerface device 14 and is dedicated to sensor I/O and force feedback(if provided) of interface device 14. Microprocessor 26 can be providedwith software instructions to wait for commands or requests fromcomputer host 12, decode the command or request, and handle/controlinput and output signals according to the command or request. Inaddition, processor 26 preferably operates independently of hostcomputer 12 by reading sensor signals and,; if appropriate, calculatingappropriate forces in accordance with a host command. Suitablemicroprocessors for use as local microprocessor 26 include theMC68HC711E9 by Motorola, the PIC16C74 by Microchip, and the 82930AX byIntel Corp., for example. Microprocessor 26 can include onemicroprocessor chip, or multiple processors and/or co-processor chips,and/or digital signal processor (DSP) capability.

Microprocessor 26 can receive signals from sensors 28 and providesignals to actuators 30 of the interface device 14 in accordance withinstructions provided by host computer 12 over bus 24. For example, in apreferred local control embodiment, host computer 12 provides high levelsupervisory commands to microprocessor 26 over bus 24, andmicroprocessor 26 manages low level force control loops to sensors andactuators in accordance with the high level commands and independentlyof the host computer 12. The force feedback system thus provides a hostcontrol loop of information and a local control loop of information in adistributed control system. This operation is described in greaterdetail in U.S. Pat. No. 5,734,373, incorporated herein by reference.Microprocessor 26 can also receive commands from any other input devices39 included on interface apparatus 14, such as buttons, and providesappropriate signals to host computer 12 to indicate that the inputinformation has been received and any information included in the inputinformation. Local memory 27, such as RAM and/or ROM, is preferablycoupled to microprocessor 26 in interface device 14 to storeinstructions for microprocessor 26 and store temporary and other data.In addition, a local clock 29 can be coupled to the microprocessor 26 toprovide timing data.

Sensors 28 sense the position, motion, and/or other characteristics of auser object 34 of the interface device 14 along one or more degrees offreedom and provide signals to microprocessor 26 including informationrepresentative of those characteristics. Rotary or linear opticalencoders, analog potentiometers, photodiode or photoresistor sensors,velocity sensors, acceleration sensors, strain gauge, or other types ofsensors can be used. Sensors 28 provide an electrical signal to anoptional sensor interface 36, which can be used to convert sensorsignals to signals that can be interpreted by the microprocessor 26and/or host computer system 12.

Actuators 30 can be included in some interface devices to transmitforces to manipulandum 34 or housing of the interface device 14 in oneor more directions along one or more degrees of freedom in response tosignals received from microprocessor 26. Actuators 30 can include twotypes: active actuators and passive actuators. Active actuators includelinear current control motors, stepper motors, pneumatic/hydraulicactive actuators, a torquer (motor with limited angular range), voicecoil actuators, and other types of actuators that transmit a force tomove an object. Passive actuators can also be used for actuators 30,such as magnetic particle brakes, friction brakes, orpneumatic/hydraulic passive actuators. Actuator interface 38 can beoptionally connected between actuators 30 and microprocessor 26 toconvert signals from microprocessor 26 into signals appropriate to driveactuators 30.

Other input devices 39 can optionally be included in interface device 14and send input signals to microprocessor 26 or to host processor 16.Such input devices can include buttons, dials, switches, levers, orother mechanisms. For example, in embodiments where user object 34 is ajoystick, other input devices can include one or more buttons provided,for example, on the joystick handle or base. Power supply 40 canoptionally be coupled to actuator interface 38 and/or actuators 30 toprovide electrical power. A safety switch 41 is optionally included ininterface device 14 to provide a mechanism to deactivate actuators 30for safety reasons.

Manipulandum (“end effector” or “user object”) 34 is a physical object,device or article that may be grasped or otherwise contacted orcontrolled by a user and which is coupled to interface device 14. By“grasp”, it is meant that users may releasably engage, contact, or gripa portion of the manipulandum in some fashion, such as by hand, withtheir fingertips, or even orally in the case of handicapped persons. Theuser 22 can manipulate and move the object along provided degrees offreedom to interface with the host application program the user isviewing on display screen 20. Manipulandum 34 can be a joystick, mouse,trackball, stylus (e.g. at the end of a linkage), steering wheel,sphere, medical instrument (laparoscope, catheter, etc.), pool cue (e.g.moving the cue through actuated rollers), hand grip, knob, button, orother object. Mechanisms which may be used to provide the degrees offreedom to the user object include gimbal mechanisms, slotted yokemechanisms, flexure mechanisms, etc. Various embodiments of suitablemechanisms are described in U.S. Pat. Nos. 5,767,839, 5,721,566,5,623,582, 5,805,140, 5,825,308, and patent application Ser. Nos.08/965,720, 09/058,259, and 09/138,304, all incorporated herein byreference.

A mechanical transmission can be connected between the actuator and themanipulandum and/or between the sensor and the manipulandum. Theinvention described herein is most suitable for devices including atransmission between the sensor and the manipulandum, since thetransmission typically causes the most position slip in the device.Mechanical transmissions can include belt drive transmissions, capstan(cable) drive transmissions, and gear transmissions, many of which aredescribed in greater detail in the patents and patent applicationsincorporated above. Furthermore, other compliance issues that may occurare described in copending patent application Ser. No. 09/138,309,incorporated herein by reference.

FIG. 2 is a top plan view of one example of a interior portion 50 of aforce feedback mouse interface device 14 that can make use of thepresent invention. Mouse 54 (shown in dashed lines) is the manipulandum34 which the user grasps and moves in planar degrees of freedom, similarto a traditional mouse. A mouse device similar to mouse device 50 isdescribed in copending application Ser. Nos. 08/881,691 and 08/965,720,both incorporated herein by reference. Unlike a traditional mouse, mouse54 has hard limits to its motion in its planar workspace due to it beingattached to the mechanical linkage, described below.

Mechanical linkage 52 provides support for a mouse 54 and couples themouse to a grounded surface 56, such as a tabletop or other support.Linkage 60 is, in the described embodiment, a 5-member (or “5-bar”)linkage including a ground member 62 (the base), a first base member 64coupled to ground member 62, a second base member 68 coupled to groundmember 62, a first link member 66 coupled to base member 64, and asecond link member 70 coupled to link member 66 and base member 68.Mouse 54 is coupled to the linkage at the coupling between link members66 and 70. Ground member 62 of the linkage 60 is a base for the supportof the linkage and is coupled to or resting on ground surface 56. Feweror greater numbers of members in the linkage can be provided inalternate embodiments.

The members of linkage 60 are rotatably coupled to one another throughthe use of rotatable pivots or bearing assemblies having one or morebearings, all referred to as “bearings” herein. Base member 64 isrotatably coupled to ground member 62 by a grounded bearing 72 and canrotate about an axis A. Link member 66 is rotatably coupled to basemember 64 by bearing 74 and can rotate about a floating axis B, and basemember 68 is rotatably coupled to ground member 62 by bearing 72 and canrotate about axis A. Link member 70 is rotatably coupled to base member68 by bearing 76 and can rotate about floating axis C, and link member70 is also rotatably coupled to link member 66 by bearing 78 such thatlink member 70 and link member 66 may rotate relative to each otherabout floating axis D. In an alternate embodiment, link member 66 can becoupled at its end to a mid-portion of link member 70 and mouse 54 canbe coupled to the end of link member 70. Mouse 54 in the preferredembodiment is coupled to link members 66 and 70 by rotary bearing 78.

Linkage 60 is formed as a five-member closed-loop chain. Each member inthe chain is rotatably coupled to two other members of the chain. Thefive-member linkage is a planar linkage that is arranged such that themembers can rotate about their respective axes to provide mouse 54 withtwo degrees of freedom, i.e., mouse 54 can be moved within a planarworkspace defined by the x-y plane, which is defined by the x- andy-axes.

Sensors 82 a and 82 b collectively sense the movement of the mouse 54 inthe provided degrees of freedom and send appropriate signals to theelectronic portion of interface device 14. Sensor 82 a senses movementof link member 68 about axis A, and sensor 82 b senses movement of basemember 64 about axis A. These sensed positions about axis A allow thedetermination of the position of mouse 54 using known constants such asthe lengths of the members of linkage 60 and using well-known coordinatetransformations. Sensors 82, as described, use gear transmissions andthus can be subject to slip with respect to the manipulandum. Thesensors are described in greater detail below with respect to FIG. 3.

Actuators 84 a and 84 b to transmit forces to mouse 54 in space, i.e.,in two (or more) degrees of freedom of the user object. In the preferredembodiment, actuators 84 a and 84 b are electromagnetic voice coilactuators which provide force through the interaction of a current in amagnetic field. These types of actuators are described in detail inpatent application Ser. Nos. 08/560,091, 08/881,691, and 08/965,720, allincorporated by reference herein. In other embodiments, other types ofactuators can be used, both active and passive, such as DC motors,pneumatic motors, passive friction brakes, passive fluid-controlledbrakes, etc.

The housing plate of a grounded portion of actuator 84 a is rigidlycoupled to ground member 62 (or grounded surface 56) and a movingportion of actuator 84 a (preferably a coil) is integrated into the basemember 64. The actuator 84 a transmits rotational forces to base member64 about axis A. The housing plate of the grounded portion of actuator84 b is rigidly coupled to ground member 62 or ground surface 56 throughthe grounded housing of actuator 84 b, and a moving portion (preferablya coil) of actuator 84 b is integrated into base member 68. Actuator 84b transmits rotational forces to link member 68 about axis A. Thecombination of these rotational forces about axis A allows forces to betransmitted to mouse 54 in all directions in the planar workspaceprovided by linkage 60 through the rotational interaction of the membersof linkage 60.

Base member 64 includes an integrated coil portion 90 a on which a wirecoil 92 a is provided. Wire coil 92 a includes at least two loops ofwire wound on a portion 90 a, e.g. 222 loops, in the describedembodiment, are wound like a spool about a center portion of portion 90a. Terminals (not shown) from wire coil 82 a to the electronic portionof the interface device are provided so that host computer 18 or localmicroprocessor 26 can control the direction and/or magnitude of thecurrent in wire coil.

Voice coil actuator 84 a also includes a magnet assembly 98 a, which isgrounded and preferably includes four magnets 100 a and a flux plate 102a. Opposite polarities of magnets 100 face each other, such that coil 92a is positioned between opposing polarities on either side of the coil.The magnetic flux guide surrounding the magnets is provided as, in thedescribed embodiment, metal plate 102 a provided on the top side of themagnets 100 a and metal base plate 85 provided on the bottom side of theactuator 84 a. Plates 102 a and 85 house actuator 84 a to allow magneticflux from magnets 100 a to travel from one end of the magnets 100 a tothe other end.

The magnetic fields from magnets 90 a interact with a magnetic fieldproduced from wire coil 82 a when current is flowed in coil 82 a,thereby producing forces on member 64. The magnitude or strength of theforce is dependent on the magnitude of the current that is applied tothe coil, the number of loops in the coil, and the magnetic fieldstrength of the magnets. The direction of the force depends on thedirection of the current in the coil; the force can be applied in eitherdirection about axis A. Force can thus be applied to member 64 andthrough member 66, thereby applying force to mouse 54 in the x-y planeworkspace of the mouse. Stop supports 91 can be provided at each end ofthe range of motion of the member 64 about axis A to provide a limitedangular range.

Voice coil actuator 64 b operates similarly to actuator 64 a. A currentis flowed through coil 82 b to cause interaction with a magnetic fieldfrom magnets 100 b of magnet assembly 98 b which is similar to themagnet assembly 98 a described above, and inducing magnetic forces thatrotate portion 90 b of base member 68 about axis A. This causes forcesto be applied to mouse 54 in the x-y workspace of the mouse through themember 68 and member 70.

In alternate embodiments, the mechanical linkage 60 can be replaced byother mechanical linkages or structures which can provide desireddegrees of freedom. For example, linear motion can be provided. In otherembodiments, other types of actuators may be used in place of or inaddition to actuators 84 of the interface device. For example, thelinkage can be driven by a direct drive DC motor or a geared/belt DCmotor to provide mechanical advantage.

Sensing Position Slip in Interface Devices

In some interface devices, slip may occur between the sensor and themanipulandum, where the manipulandum is moved and the sensor does notdetect or detects inaccurately the movement of the manipulandum in oneor more degrees of freedom of the manipulandum. Slip is often a greaterproblem when a mechanical transmission is provided between the sensorand the manipulandum. Mechanical transmissions are typically provided insuch a configuration to amplify the motion of a moving sensor member(such as an encoder wheel) for a given amount of motion of themanipulandum, thereby increasing the sensing resolution of the sensor.Examples of such mechanical transmissions include gear drives, beltdrives, and capstan drives.

Slip can cause problems in the display of graphical objects orinteractions on a display screen of computer device, since the hostcomputer controlling the display assumes the manipulandum is in aparticular position when it may actually be in a different position; thedifference is due to the slip. Furthermore, with respect to forcefeedback interface devices, many types of force sensations output by thedevice, such as spring forces, damping forces, attractive forces,texture forces, periodic forces activated based on position, etc.,depend at least in part on the position of the manipulandum and can bedistorted to varying degrees if slip occurs.

The present invention detects slip so as to reduce or eliminate theseundesired effects. One aspect of the present invention for detecting theslip in a device uses a reference sensor (or “index sensor”) that isconnected directly to the manipulandum. In a simple case, this sensorcan provide a digital flag signal that changes state when themanipulandum travels past a predetermined location (or one of severaldifferent locations) in its range of travel or workspace. The importantaspect of the reference sensor is that it is connected to themanipulandum “before” any mechanical transmission, i.e. there is notransmission mechanism between the reference sensor and themanipulandum. It is therefore not possible to generatetransmission-induced slippage between the manipulandum and the referencesensor. In general, there is preferably one reference sensor for eachdegree of freedom of the manipulandum that is sensed on the device.

One example of a reference sensor is an emitter/detector pair similar tothose used in standard optical encoders. This sensor can be used, forexample, to detect the edges of a “flag,” e.g., a reference locationsuch as a mark, edge, or other distinguishing feature, as it moves pastthe sensor position. Multiple flags can be used to generate severalpossible different index locations that are sensed during the motion ofthe manipulandum.

FIG. 3 is a perspective view illustrating one embodiment of thereference sensor of the present invention in use with the force feedbackmouse embodiment shown in FIG. 2. A mechanical member 101 is directlyconnected to the manipulandum with no mechanical transmission positionedtherebetween. Member 101, for example, is shown as the end of amechanical member 64 or 68 of the force feedback mouse device shown inFIG. 2, in which a wire coil is coupled to the member to pass through amagnetic field and generate a force on the manipulandum. Member 101rotates about an axis A as shown by arrows 107.

At one end of member 101 is sensor 62 a or 62 b as shown in FIG. 2,which includes a gear transmission mechanism 103. Mechanism 103 includesteeth 104 coupled to the member 101 and a gear 106 which includes gearteeth about its circumferential surface that interlock with the teeth104. When member 101 moves, gear 106 thus rotates about axis E. Gear 106is rigidly coupled to an encoder disk 108 by a shaft 110, where theencoder disk includes a number of marks or slots 112 that rotate throughone or more emitter-detectors 114. The emitter-detectors 114 sense themotion of member 101 by detecting the passage of the marks or slots(“counts”), as is well known to those skilled in the art. Gear 106 canbe biased to engage the teeth 104 by a pivotable spring-loaded member116 which presses the gear 106 to engage the teeth 104 and pivots abouta grounded coupling 105.

A reference sensor of the present invention can take many forms. Oneembodiment provides a grounded emitter 120 and a grounded detector 122positioned on opposite sides of the member 101 somewhere in the range ofmotion of the member, such as approximately in the center of the rangeof motion. The emitter 120 emits a beam 124 that is blocked by themember 101 from reaching the detector 122. An aperture 126 is placed inthe member 101, the aperture extending the entire thickness t of themember and allowing the beam 124 to reach the detector 122 when themember 101 is in a position placing the aperture 126 directly in thepath of the beam 124. Thus, the detector will detect a change in state(producing a signal edge) three times during a motion in a singledirection, assuming the member 101 starts in a position completelyoutside the beam: when one (or either) outside edge or side 127 of themember 101 moves to block the beam, when a first-encountered physicaledge of the aperture 126 allows the beam to be transmitted, and when thesecond-encountered physical edge of the aperture 126 again blocks thebeam. In other embodiments, a different number of changes in state canbe detected; for example, only the side of the member 101 need bedetected.

Alternatively, other reference sensors can be used. For example, one ormore members can be placed extending out from the member 101, where theextending members successively moves through an emittedhorizontally-aligned beam of an optical sensor as the member 101 moves.Other types of optical sensors, such as phototransistor, photodiode, andphotoresistor sensors, can be used. Magnetic sensors can also be used insome embodiments.

It should be noted that, in other embodiments, other types of interfacedevices can be used with the reference sensor of the present invention.For example, interface devices such as a joystick, trackball, rotatablecylinder, steering wheel, gamepad, or other types of mice can be used.For example, a joystick handle can include a member extending from itslower section which crosses a beam of an emitter-detector. The closerthe sensor is positioned to the actual manipulandum, the more accurateis the reference position sensed as an indication of actual manipulandumposition.

FIG. 4 is a flow diagram illustrating a first method 200 of the presentinvention of sensing motion of a manipulandum of an interface devicewhile compensating for slippage between sensor and manipulandum. Thismethod can be performed by program code running on the localmicroprocessor, host computer, or other controlling device.

The process begins at step 202. In a step 204, a device position delta(change from the last position) is determined from the position sensor.This is obtained by reading a current device position and finding thedifference between the current position and the last sensed deviceposition. In step 206, the process checks whether a slip in position hasbeen detected by using the reference sensor(s) described above. Thedetails of determining if a slip has occurred are detailed below withrespect to FIG. 5. This step also preferably provides the amount of slipthat has occurred. If slip has not been detected, then in step 208 acomputation value is determined using the position delta determined instep 204. The “computation value” is the position value that is used fordisplaying and computation purposes, e.g. by the host computer, asopposed to the device position value which is the position of themanipulandum in its workspace. For example, a computation value can bedetermined in one embodiment by simply adding the sensed position deltato the previously-used computation value to achieve the currentcomputation value. Alternatively, the position delta can be factoredinto a more complicated computation value that depends also on otherfactors. For example, a ballistics algorithm can be used whichdetermines a computation value based on factors such as the manipulandumvelocity and the location of the manipulandum within its workspace. Someballistics methods are described in greater detail in copending patentapplication Ser. No. 08/924,462, incorporated herein by reference. Oncethe computation value is determined, the process continues to step 214where the host computer's display is updated based on the computationvalue, and other processes using the computation value can be performed(force computation, etc.). For example, if the computation value isdetermined by a device processor, the computation value can be sent tothe host computer as a cursor location which updates the display of acursor on the screen. Or, the motion of a displayed user-controlledentity or viewpoint in a game or simulation can be updated. The processthen returns to step 204.

If slip has been detected in step 206, then in step 209 the deviceupdates the device position value using the amount of slip detected instep 206, and uses this as the current device position. This correcteddevice position allows the device to accurately determine when, forexample, the manipulandum hits a limit to its workspace. In step 210,the process determines the computation value without the use of theposition delta resulting from the slip updating in step 209. The slippedposition delta from step 209 has been corrected and may include a largejump in position, and it is not desired to update the display screen inaccordance with this jump since such a jump can be disconcerting to theuser. For example, in systems where the absolute position of the deviceis being used to position an object in a graphical environment havingthe same proportional dimensions, this discontinuity can bedisconcerting for the user. If the manipulandum position is displayedgraphically as a graphical object on the display screen, then a suddenjump in the displayed location of the graphical object will occur if theslippage is included in the computation value. Furthermore, if themanipulandum position is being used in force feedback embodiments forforce computations, then a sudden, unexpected force change may occurthat is noticeable to the user. Thus, the computation value in step 208is determined without using the slippage position delta determined instep 209 to prevent any significant discontinuities from occurring incomputation value and in the display. In a simple case, this means thatthe last-computed computation value is simply used again as the currentcomputation value, i.e. the display does not change. The current deviceposition is still saved to be used as the “last” device position in thenext iteration so that the next determined position delta is determinedcorrectly.

In next step 212, if the current position of the manipulandum indicatesthat the manipulandum is at a limit to its workspace, then thecomputation value should be compensated for this event. Thiscompensation allows recovery of the situation in which the deviceposition is at one of its limits and the computation position has notreached a limit value (such as the edge of the screen). In such a case,without compensation, the user would try to continue to move the cursor(or other object) towards the edge of the screen, but could not sincethe manipulandum has encountered a hard limit to its motion. Thissituation can happen more frequently if the slip described herein ispresent in the device, since the slip increases any offset betweendevice position in its workspace and screen cursor position on thedisplay screen. Furthermore, the display from the computation value usedin step 210 may cause a greater offset. To mitigate this situation, thecomputation value can be modified to allow the user to continue to movethe cursor when the manipulandum has reached a physical limit. Forexample, in one embodiment, rate control limits are provided, in whichthe user can move the mouse into a border region of its workspace; theamount of distance moved into the border region controls the rate ofspeed of motion of the cursor in a corresponding direction. In adifferent embodiment, once the device reaches a predetermined distanceto a limit, the remaining distance the cursor must travel to reach ascreen limit is determined and an appropriate scaling factor is appliedto the motion of the cursor to allow it to reach the screen limit whenthe device limit is reached. These and other methods are described ingreater detail in copending application Ser. No. 08/924,462,incorporated herein by reference.

In next step 214, the computation value is used to update the display ofthe host computer and update other processes (force computation, etc.).The process then returns to step 204.

FIG. 5 is a flow diagram 230 illustrating the step 206 of FIG. 4 inwhich position slip is detected. The process begins at 232, and in step234, the process checks whether a signal edge is detected in the outputof the reference sensor. The signal edge indicates that the sensing of aflag or reference location has occurred. If no signal edge is sensed,then no reference location has been sensed, and the process is completeat 236. If a signal edge has been detected, then in step 238 the processimmediately reads a position value that corresponds with the signal edgefrom the position sensor and stores this position value (or the deviceposition read in step 204 of FIG. 4 can be used as the correspondingdevice position if it was read sufficiently close in time to the signaledge).

In step 240, the process determines which reference location in therange of motion of the member 101 is being sensed. For example, threedifferent reference locations are sensed in the embodiment of FIG. 3,including one side of the member 101 and the two sides of the aperture126. Each reference location is associated with a stored referenceposition value. To determine which of these reference locations wassensed, different methods can be used. In one method, the referencesensor can detect 2 different types of signal edges (rising or fallingedge), and the process can use the information of which type of edge wasdetected to determine which reference location is being sensed. Thus,the first encountered reference location may produce a rising edge, thesecond location may produce a falling edge, and the third encounteredlocation may produce a rising edge. The process can further examine thecurrent direction of member 101 to help determine which referencelocation was sensed, e.g. the reference locations are encountered in aparticular order depending on the direction of travel of the member 101.

In next step 242, the process checks if it is the first time that areference location has been sensed. If it is the first time, then theposition value read at step 238 is stored at step 244 to correspond tothe identified reference location and the process is complete at 236. Inaddition, a determination of no slippage is assumed at this stage andreturned to the main process of FIG. 4. Since there is only one positionvalue known to correspond with the reference location, no comparisonwith previous positions at that location can be made, so that noslippage can be determined. In one preferred embodiment, a number ofposition values are stored for the first few times that signal edges arefirst detected before slippage is computed so that a stable, accuratereference location position is obtained. For example, the first positionvalue that does not significantly deviate after five consecutivesensings is established as the position value for that referencelocation, and only after that position is established does the processcheck for slippage starting at step 246.

If it is not the first time that the reference location has been sensed(or the position value for that reference location is otherwiseestablished), then in step 246 the process compares the current positionvalue to the most recent, previously stored position value for thisreference location. In step 248, the slip amount is determined as thedifference between these position values. If the slip value is zero,then it indicates that no slippage has occurred. If the slip value is anonzero value, then some slippage has occurred, since a single referencelocation has been sensed at a different position of member 101. In step250, the process checks if the slip amount is greater than apredetermined threshold; this threshold should at least be larger thanthe possible variation in the reference sensor position produced by thesensor device being used. If the slip amount is less than the threshold,the slip amount is too low to be worth compensating, and no slip isconsidered to be detected at step 252. The process is then complete at256. If the slip amount is greater than the threshold in step 250, thenin step 254 then slip is considered to have been detected, and allstored reference position values are updated with the slip amount. Sinceslip has occurred and the reference locations are now being detected ata new position, the stored reference positions for those locations mustbe updated so that any additional slip can be detected if it occurs. Forexample, the position values corresponding to all three referencelocations in the embodiment of FIG. 3 are all updated by adding the slipamount to each position value; this prevents the same slip from beingdetected again when the next reference position on member 101 isdetected. The process is then complete at 256.

A special case exists when the mechanical transmission that is used inthe device skips only in discrete units. An example of this case is agear drive transmission, such as shown in FIG. 3. When a transmissionsuch as this is employed, it is known that any slippage that occurs inthe system will be equivalent to a change of one discrete unit of thetransmission. In a geared system, this means that any observed slipswill always be a certain number of gear teeth that have been jumpedover. Since the number of position sensor counts that occur for eachgear position is fixed (call this value N), all actual slip in thesystem will be in multiples of the value N. Since N is typically severalsensor counts (when using a digital encoder sensor), this allows thedetection of slip to be easier and the updating of reference positionvalues to be more accurate.

FIG. 6 is a flow diagram illustrating a second method 280 of the presentinvention of sensing motion of a manipulandum of an interface devicewhile compensating for slippage between sensor and manipulandum. In thismethod, slippage is not ignored for computation value purposes, so thatthe screen display is correlated with slipped device position smoothlyand the offset between device position and screen display is correctedover time. Basically, small “corrections” are applied to the deviceposition delta values in order to reduce slippage error. Thesecorrections are then also subtracted out of the error value. Thisprocess is continued until the error values are reduced to zero and theactual manipulandum position matches the sensed manipulandum position.The result is that the computation value slowly approaches the correctvalue as the error values are reduced. This allows this second method tobe used in embodiments that are not using rate control borders, scaling,or other methods that allow a user to continue to control a cursor aftera device limit has been reached. This method can be more suitable fordevices providing absolute position mapping with no great changes tocomputation position, such as in most joystick embodiments.

The process begins at 282, and in step 284, the process determines thedevice position delta, similar to step 204 of FIG. 4. In step 286, theprocess determines if slip has occurred, similar to step 206 of FIG. 4.If slip has occurred, then in step 288 an error value is set equal tothe previous error value plus the current slip amount found in step 286(the error value is initially set to zero upon device start-up). Theerror value is thus a total indication of slip that has occurred duringdevice operation, including slip amounts from previous slippageoccurrences and adjustments from manipulandum movement (describedbelow). After step 288, or if no slip has been detected in step 286,then step 290 is performed, in which the process checks whether theerror value is currently is nonzero. If not, i.e. if the error value iszero, then no slippage needs to be compensated for and in step 298 theposition delta from step 284 is used to determine the computation valuewithout correction. The computation value can then be used for displaypurposes, force computation purposes, etc. The process then returns tostep 284.

If the error value is nonzero in step 290, then in step 292 the processchecks whether the manipulandum (or member 101) is moving. This check isperformed so that computation value corrections do not causedisconcerting effects to the user that may occur if the manipulandum isnot moving, e.g. the mouse may be stationary yet the cursor may bemoving to correct for slippage. To avoid this, computation valuecorrections are preferably made only when the manipulandum is moving.Thus, if the manipulandum is moving in step 292, the process continuesto step 298 to determine a computation value normally. If themanipulandum is moving, the process continues to step 294 to adjust theposition delta (described below). In some embodiments, to allowcomputation value corrections when a device limit has been reached (andthe manipulandum is stationary), a check can be made for the amount oftime that the manipulandum has been stationary. For example, if themanipulandum is stationary for a predetermined number of iterations,then the manipulandum can be considered moving. A check can also be madewhether the manipulandum is positioned at a device limit.

In step 294, the position delta determined in step 284 is adjusted basedon the error value. This can be implemented in many ways. For example,in one embodiment, the position delta can be adjusted by adding orsubtracting a predetermined number of counts (or other sensormeasurement units) to the current device position, thereby changing theposition delta, to compensate for the slippage. The number of counts ispreferably small enough so that the adjustment is not easily noticed bythe user. This may cause the slippage error to be fully corrected onlyafter multiple iterations of the process 280, since the error valuecould be much larger than the adjustment made. However, such slowcorrection is desired to prevent the user from noticing the correctionprocess.

Alternatively, the adjustment can be proportional to the size of thecurrent error value, where larger error values cause larger adjustments(with a maximum limit to the size of the adjustment so that the userwill not easily notice the adjustments). In some embodiments, eachadjustment or correction in the loop of method 280 can depend on thecurrent manipulandum position in its workspace. For example, if themanipulandum is moved by the user to an actual position that wouldincrease the slippage amount (if the incorrect sensed position were heldstationary), then a greater adjustment can be applied. If, however, themanipulandum is moved by the user to an actual position that woulddecrease the slippage amount (if the incorrect sensed position were heldstationary), then a smaller correction can be applied, since themanipulandum motion is already helping to decrease the slippage error bybeing in the direction of decreasing the slippage delta.

After step 294, the process adjusts the error value by the amount of theadjustment made in step 294. Thus, if 10 counts were added to theposition delta in step 294, the error value can be reduced by 10 counts.The process then continues to step 298 to determined a computation valuethat is based on the adjusted position delta. This allows the graphicaldisplay (and other computations based on device position) to becorrelated with a device position that is slowly being corrected toremove slippage error. At some iteration, the error value has beenreduced in size sufficiently so that the last adjustment at step 296causes the error value to be zero, and slippage error has beeneliminated from the system.

While this invention has been described in terms of several preferredembodiments, it is contemplated that alterations, permutations andequivalents thereof will become apparent to those skilled in the artupon a reading of the specification and study of the drawings. Forexample, many different types of interface devices, both force feedbackand non-force feedback, may be used with the position sensing techniquesdescribed herein. Furthermore, certain terminology has been used for thepurposes of descriptive clarity, and not to limit the present invention.It is therefore intended that the following appended claims includealterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

1. A method for correcting for slippage in a position sensing system ofan interface device coupled to a host computer, the method comprising:determining a device position delta from a sensed position of amanipulandum of said interface device in a degree of freedom, saidposition delta indicating a change in position of said manipulandum;determining if position slip has occurred in said position sensingsystem, said position slip caused by a change in position of saidmanipulandum that was not sensed by a sensor of said interface device;and correcting an error in said sensed position caused by said positionslip by adjusting said sensed position to take into account saidposition slip, and using said adjusted position delta as said positionof said manipulandum.
 2. A method as recited in claim 1 wherein saiddetermining if said slip has occurred includes checking a referencesignal from a reference sensor included on said interface device, saidreference sensor detecting a position of a reference location coupled tosaid manipulandum.
 3. A method as recited in claim 1 further comprisingdetermining a computation value from said device position delta, whereina display of said host computer is updated based on said computationvalue.
 4. A method as recited in claim 3 wherein said computation valueis not adjusted with said device position delta when said position slipis detected.
 5. A method as recited in claim 4 wherein a force having amagnitude and direction is determined at least partially based on saidcomputation value, and wherein said force is output by an actuatorincluded in said interface device.
 6. A method as recited in claim 3wherein said determining a device position, said determining if aposition slip has occurred, and said correcting an error are performedover a plurality of iterations, and wherein said sensed position isadjusted by an amount which gradually reduces said error in said sensedposition over said plurality of iterations.
 7. A method as recited inclaim 6 wherein said adjusting of said sensed position is not performedif said manipulandum is not moving in said degree of freedom.
 8. Amethod as recited in claim 1 wherein said position slip is caused by amechanical transmission provided between said sensor and saidmanipulandum.
 9. A method as recited in claim 1 wherein said mechanicaltransmission includes one of a gear drive, a belt drive, and a capstandrive.
 10. A method as recited in claim 1 wherein said computation valueis modified to compensate for an offset between said device position anda screen position of a user controlled object displayed by said hostcomputer.
 11. A method as recited in claim 3 further comprisingmodifying said computation value to allow a user-controlled objectdisplayed by said host computer to continue to be moved after saidmanipulandum has reached a limit to a workspace of said manipulandum,said user-controlled object continuing to be moved in a directioncorresponding to a direction of said manipulandum into said limit. 12.An interface device providing input to a host computer based onmanipulations of a user, said interface device comprising: amanipulandum physically contacted by said user and movable in at leastone degree of freedom; a position sensor that detects a position of saidmanipulandum in at least one of said degrees of freedom; a mechanicaltransmission coupled between said manipulandum and said position sensor;and a reference sensor that detects the motion of at least one referencelocation coupled to said manipulandum, said reference sensor providing areference signal used to determine if position slip has occurred betweensaid manipulandum and said position sensor.
 13. An interface device asrecited in claim 11 wherein said mechanical transmission includes a geardrive.
 14. An interface device as recited in claim 11 wherein saidmechanical transmission includes a belt drive.
 15. An interface deviceas recited in claim 11 wherein said position sensor is a relativedigital encoder.
 16. An interface device as recited in claim 11 whereinat least one of said reference locations is a physical edge to a member,and wherein said reference sensor is an optical sensor that detects apassage of said physical edge through an transmitted beam.
 17. Aninterface device as recited in claim 11 wherein said reference sensorprovides a rising signal edge or a falling signal edge in said referencesignal to indicate that one of said reference locations has beendetected.
 18. An interface device as recited in claim 11 furthercomprising a local microprocessor, said local microprocessor receivingsaid reference signal and adjusting said detected position of saidmanipulandum if said position slip has occurred.
 19. An interface deviceas recited in claim 18 wherein said local microprocessor determines acomputation value from a position delta determined from said position,wherein said computation value is adjusted if said position slip hasoccurred, said computation value provided to said host computer toupdate a display of said host computer.